Transketolase

ABSTRACT

A protein having transketolase activity, containing an amino acid sequence which is a part sequence of at least 100 amino acids from SEQ ID NO 2, and nucleic acids coding for this protein and its use are described.

BACKGROUND OF THE INVENTION

The present invention relates to proteins having transketolase activity, their use in test systems, and nucleic acids which code for these proteins.

Plants are able to synthesize organic compounds from atmospheric carbon dioxide using light energy with formation of oxygen. This process is called photosynthesis.

It is to be assumed that the efficient formation, utilization and distribution of the photosynthesis products severely affect the growth of a plant.

As plants are dependent on a functioning photosynthesis and comparable reactions do not occur in animal organisms, the photosynthesis apparatus presents itself as an ideal target for the use of herbicides.

The complex reactions which lead to carbon dioxide fixation are divided into light and dark reactions. The light reaction is used for making available energy in the form of ATP and reduction equivalents in the form of NADPH. In the dark reaction (reductive pentose phosphate cycle or Calvin cycle), these compounds are used for the synthesis of organic carbon compounds.

Some of the known herbicides (eg. dichlorophenylmethylurea or paraquat) act by inhibition of the light reaction. The dark reaction is not utilized as a point of attack for herbicides.

The enzyme reactions of the reductive pentose phosphate cycle are divided into three sections:

a) carboxylation

b) reduction

c) regeneration.

In carboxylation, carbon dioxide reacts with the acceptor molecule ribulose bisphosphate (RuBP), whereby two molecules of 3-phosphoglycerate (3-PGA) are formed. After phosphorylation, 3-PGA is then reduced to glyceraldehyde-3-phosphate (GAP). In the regeneration phase, the acceptor molecule RuBP is resynthesized from the GAP formed. Of six molecules of GAP formed, one molecule can be employed for other metabolic pathways.

A multiplicity of the enzymes involved in the reductive pentose phosphate cycle are potential points of attack for herbicides. Plastid transketolase, however, assumes a special position. Like transaldolase, transketolase (E.C. 2.2.1.1.) catalyzes two reactions:

(1) fructose-6-phosphate+glyceraldehyde-3-phosphate→ erythrose-4-phosphate+xylulose-5-phosphate

(2) sedoheptulose-7-phosphate+glyceraldehyde-3-phosphate→ ribose-5-phosphate+xylulose-5-phosphate

The substrates and products involved in the reactions represent points of linkage between the reductive pentose phosphate cycle and other metabolic pathways. Exported triose phosphates are used in the cytoplasm as substrates for glycolysis and gluconeogenesis. Fructose-6-phosphate is used as a precursor molecule for the preparation of starch in the plastids. Erythrose-4-phosphate is an intermediary between primary and secondary metabolism. Linked with phosphoenol pyruvate, erythrose-4-phosphate opens in the Shikimate pathway, which leads to the synthesis of aromatic amino acids and phenolic substances.

Ribose-5-phosphate is used as a substrate in different metabolic pathways.

In plant tissues, two transketolase isoforms were described which differ in their subcellular compartmentalization (Murphy and Walker, 1982, Planta 155, 316-320).

The plastid transketolase is responsible in green tissues for more than 75% of the total activity. The active enzyme is present as a homotetramer (holoenzyme) having a relative molecular weight of 150 kDa. As cofactors, transketolase needs vitamin B₁ (thiamine pyrophosphate) and magnesium. In the absence of thiamine pyrophosphate or in the presence of mercaptoethanol, the tetramer dissociates into two dimers (apoenzymes) having a relative molecular weight of 74 kDa each. Holo- and apoenzyme are catalytically active, the holoenzyme having a substantially higher activity than the apoenzyme.

DESCRIPTION OF THE PRIOR ART

Genes which code for transketolase were previously isolated from Saccharomyces cerevisiae (Flechter et al., Biochemistry 31, (1993), 1892-1896; Sundstrom et al., J. Biol. Chem. 268, (1993), 24346-24352; Schaff-Gerstenschlager et al., Eur. J. Biochem. 217, (1993), 487-492), from Hansenula polymorpha (Janowicz et al., Nucl. Acids Res. 13, (1985), 3043-3062), human erythrocytes (Abedinia et al., Biochem. Biophys. Res. Commun. 183, (1992), 1159-1166; McCool et al., J. Biol. Chem. 268, (1993), 1397-1404), Rhodobacter sphaeroides (Chen et al., J. Biol. Chem. 266, (1992), 20447-20452) and Escherichia coli (Sprenger, Biochem. Biophys. Acta 1216, (1992), 307-310; Tida et al., J. Bacteriol. 175, (1993), 5375-5383) and described. Genes of plant transketolases were unknown until now.

SUMMARY OF THE INVENTION

It is an object of the present invention to make available a plant transketolase in pure form by cloning the corresponding gene.

We have now found that this object is achieved by a protein having transketolase activity, containing the amino acid sequence shown in SEQ ID NO 2.

The amino acid sequence shown in SEQ ID NO 2 is based on the translation of the CDNA sequence shown in SEQ ID NO 1.

The amino acid sequence shown in FIG. 4 as TK-23 is the same as SEQ ID NO 2.

The amino acid sequence shown in FIG. 4 as TKL1 is depicted in SEQ ID NO 3.

The amino acid sequence shown in FIG. 4 as TKL2 is depicted in SEQ ID NO 4.

The nucleic acid sequence shown in FIG. 6 as 1-MCS: left polylinker sequence is depicted in SEQ ID NO 5.

The nucleic acid sequence shown in FIG. 6 as r-MCS: right polylinker sequence is depicted in SEQ ID NO 6.

The nucleic acid sequence shown in FIG. 9 as "A:" is depicted in SEQ ID NO 7.

The nucleic acid sequence shown in FIG. 9 as "B:" is depicted in SEQ ID NO 8.

The nucleic acid sequence shown in FIG. 9 as "C:" is depicted in SEQ ID NO 9.

The protein shown in SEQ ID NO 2 is a precursor protein consisting of 743 amino acids. The mature protein is obtainable from the precursor form by removing the chloroplastid transit peptide, which according to a computer analysis consists of the 77 N-terminal amino acids.

Both the precursor protein and proteins derived therefrom by substitution, deletion or insertion of amino acids which still have a transketolase activity belong to the proteins according to the invention.

Substitution is understood as meaning the replacement of one or more amino acids by one or more other amino acids. Preferably, conservative replacements are carried out in which the replaced amino acid has similar properties to the original amino acid, for example replacement of Glu by Asp, Val by Ile, and Ser by Thr.

Deletion is the replacement of an amino acid by a direct bond; preferred positions for deletions are the termini of the polypeptide and the linkages between the individual protein domains.

Insertions are insertions of amino acids into the polypeptide chain, in which a direct bond is formally replaced by one or more amino acids.

Particularly preferred proteins are those which are formed from SEQ ID NO 2 by N-terminal shortening by from 20 to 100 amino acids.

The invention further relates to nucleic acids which code for the abovementioned proteins. Suitable nucleic acid sequences are obtainable by back-translation of the polypeptide sequence according to the genetic code. Preferably, those codons are used for this purpose which are frequently used according to the organism-specific codon usage. The codon usage can easily be determined with the aid of computer analyses of other known genes of the organism concerned.

If the plant transketolase is to be expressed, for example, in a bacterium, it is often advantageous to use the codon usage of the bacterium in the back-translation.

The invention further relates to vectors which contain the nucleic acids coding for the transketolase according to the invention together with functional regulation signals.

These are to be understood as meaning, for example, signals for transcription and translation such as promoters and ribosome binding sites or sequences necessary for replication or integration.

The proteins according to the invention are particularly suitable for the identification of herbicidal active compounds, in particular for the discovery of transketolase-specific inhibitors.

To this end, the proteins can be employed, for example, in an enzyme test in which the activity of the transketolase is determined in the presence and absence of the active compound to be tested. From comparison of the two activity determinations, a qualitative and quantitative conclusion can be made about the inhibitory behavior of the active compound to be tested.

Using the test system according to the invention, a multiplicity of chemical compounds can be investigated for herbicidal properties rapidly and simply.

The invention further relates to herbicides which are identifiable using a test system described above.

The invention additionally consists in a process for preparing herbicides which inhibit a plant transketolase, which comprises investigating known chemical compounds in a test process described above and formulating as herbicides those having inhibitory action using customary carriers and auxiliaries.

The fact that the transketolase-inhibiting properties of a substance alone are not adequate for suitability as a herbicide, but still further tests have to be carried out is familiar to any person skilled in the art.

The process makes possible, however, the reproducible selection from a large number of substances of specifically those having high potency in order subsequently to carry out using these substances further in-depth tests familiar to the person skilled in the art.

The invention is further illustrated by the following examples.

EXAMPLES

A. Recombinant DNA methods on which Exemplary Embodiments B are based:

1. General cloning methods

Cloning methods such as eg. restriction cleavages, agarose gel electrophoresis, purification of DNA fragments, transfer of nucleic acids to nitrocellulose and nylon membranes, linkage of DNA fragments, transformation of E. coli cells, culture of bacteria, replication of phages and sequence analysis of recombinant DNA were carried out as described in Sambrook et al. (1989) (Cold spring Harbor Laboratory Press: ISBN 0-87969-309-6). Transformation and culture of Pichia pastoris was carried out according to the details of the marketing company (Invitrogen Corporation). The transformation of Agrobacterium tumefaciens was carried out according to the method of Hofgen and Willmitzer (Nucl. Acid Res. (1988) 16, 9877). The culture of agrobacteria was carried out in YEB medium (Vervliet et al. J. Gen. Virol. (1975) 26, 33).

2. Generation of cDNA libraries

For preparation of leaf-specific cDNA libraries, total RNA from tobacco leaves was isolated according to a method described by Logemann et al. (Anal. Biochem. (1987) 163,21). The poly(A)-RNA was then purified on oligo(dT)-cellulose type 7 (Pharmacia, Freiburg) according to the details of the manufacturer. After photometric concentration determination, 5 μg of the RNA thus obtained were employed for the cDNA synthesis. All chemicals and enzymes necessary for the preparation of the cDNA were supplied by the company Stratagene (La Jolla Calif. 92037, USA). The methods used were carried out according to the details of the manufacturer. The synthesis of the first and second strand of the cDNA was carried out using the ZAP-cDNA synthesis kit. The double-stranded cDNAs obtained were then provided with EcoRI-NotI adaptors and cloned in an EcoRI-cleaved Lambda ZAPII vector. After in vitro packing (Gigapack II packing extract) of the recombinant lambda DNA, XL-1 E. coli cells (Stratagene) were transformed. By counting the plaques formed the titer of the cDNA libraries was determined.

3. Inspection of a cDNA library by means of heterologous DNA probes

2×10⁵ recombinant lambda phages (Lambda ZapII) of a leaf-specific cDNA library from tobacco (variety Samsun NN) were plated out on agar plates. The phage DNA was transferred by means of standard methods (Sambrook et al. (1989); Cold Spring Harbor Laboratory Press: ISBN 0-87969-309-6) to nylon membranes (Hybond N, Amersham Buchler) and fixed to the filters by incubation for 2 hours at 80° C. The hybridization probes used were DNA fragments which were radiolabeled using a multiprime DNA labeling system (Amersham Buchler) in the presence of α-³² P-dCTP (specific activity 3000 Ci/mmol) according to details of the manufacturer. Hybridization of the membrane was carried out after prehybridization at 42° C. in PEG buffer (Amasino (1986) Anal. Biochem. 152, 304-307) for 12-16 hours. The filters were then washed at 42° C. for 3×20 minutes in 2×SSC, 0.1% SDS. Positively hybridizing phages were visualized by autoradiography and purified by standard techniques.

4. Sequence analysis of recombinant DNA

The sequencing of recombinant DNA molecules was carried out using an automatic laser fluorescence DNA sequencer (A.L.F.) from Pharmacia using fluorescence-labeled oligonucleotides according to the method of Sanger (Sanger et al. (1977) Proc. Natl. Acad. Sci. USA 74, 5463-5467).

5. Bacterial strains and yeast strains

E. coli (XL-1 Blue) bacteria were supplied by Stratagene. The Agrobacterium tumefaciens strain employed for the plant transformation (CS8Cl containing the plasmid pGV 3850kan) was described by Debleare et al. (1985, Nucl. Acid Res. 13, 4777). Pichia pastoris strain GS115 was supplied by Invitrogen Corporation (San Diego, Calif. 92121, USA).

6. Tobacco transformation

For the transformation of tobacco plants (Nicotiana tabacum L. cv. Samsun NN), 10 ml of an overnight culture of Agrobacterium tumefaciens grown under selection were centrifuged off, the supernatant was discarded, and the bacteria were resuspended in the same volume of antibiotic-free medium. Leaf disks of sterile plants (diameter about 1 cm) were bathed in this bacteria solution in a sterile Petri dish. The leaf disks in Petri dishes were then put into MS medium (Murashige and Skoog, Physiol. Plant. (1962) 15,473) containing 2% sucrose and 0.8% Bacto agar. After incubation in the dark at 25° C. for 2 days, they were transferred to MS medium containing 100 mg/l of kanamycin, 500 mg/l of claforan, 1 mg/ml of benzylaminopurine (BAP), 0.2 mg/l of naphthylacetic acid (NAA), 1.6% glucose and 0.8% Bacto agar and culturing was continued (16 hours light/8 hours darkness). Growing shoots were transferred to hormone-free MS medium using 2% sucrose, 250 mg/l claforan and 0.8% Bacto agar.

7. Analysis of total RNA from plant tissues

Total RNA from plant tissues was isolated as described in Logemann et al. (Anal. Biochem. (1987) 163,21). For the analysis, 20-40 μg of RNA were in each case separated in a formaldehyde-containing 1.5% strength agarose gel. After electrophoretic separation of the RNA molecules, the RNA was transferred to a nylon membrane by means of capillary transfer. The detection of specific transcripts was carried out as described in Amasino (Anal. Biochem. (1986) 152, 304). The cDNA fragments employed as a probe were radiolabeled using a random primed DNA labeling kit (Boehringer, Mannheim).

8. PCR amplification of nucleic acids

The PCR amplification of the transketolase for the expression of the enzyme in E. coli and Pichia was carried out in a DNA thermal cycler from Perkin Elmer. The oligonucleotides used are shown in FIG. 9. The reaction mixtures contained 1 ng of template, 0.5 μM of the corresponding oligonucleotides, 0.25 mM of nucleotides (Pharmacia), amplification buffer (16 mM (NH₄)₂ SO₄, 67 mM tris HCl (pH 8.8 at 25° C.), 0.01% Tween 20, 7.5 mM MgCl₂) and 2.5 units of the Tth DNA polymerase (Biomaster, Crottorfer Str. 25, 51109 Cologne). The amplification conditions were adjusted as follows:

    ______________________________________     Addition temperature:                        60° C.     Denaturation temperature:                        94° C.     Elongation temperature:                        72° C.     Number of cycles:  30     ______________________________________

9. Overexpression of proteins in E. coli

For the overexpression of the transketolase in E. coli, 2 ml of an overnight culture which was cultured at 28° C. were transferred to 20 ml of growth medium (LB medium completed using 10 μg/ml of tetracyclin, 200 μg/ml of ampicillin, 1 mM vitamin B1 and 1 mM MgSO₄). Growth took place at 28° C. with shaking. After 3 hours, the expression of the transketolase was induced by addition of 2 mM IPTG. The detection of the protein produced was carried out by separating the proteins in an SDS PAAG (Laemmli (1970) Nature 227, 680-685) with subsequent Coomassie staining of the proteins.

B. Exemplary embodiments

1. Cloning of the plastid transketolase

A clone which codes for transketolase was selected from a leaf-specific cDNA library from tobacco (variety Samsun NN). The DNA sequence is shown in SEQ ID NO 1.

The cDNA clone 21 of length 2629 base pairs contains an open reading frame of 2229 bases and codes for a protein having 743 amino acids. Analysis of the polypeptide using the sequence program PC/gene (submenu TRANSPEP) showed that a chloroplastid transit peptide of presumably 77 amino acids is present at the N-terminus of the protein.

2. Comparison of the plastid transketolase from tobacco with known transketolase protein sequences

Comparisons of homology of the derived amino acid sequence of the clone TK-23 (MacMolly sequence analysis program from Macintosh) with published transketolase sequences showed that the highest homologies to transketolases from Saccharomyces cervesiae exist in the area of the supposedly mature polypeptide (amino acids 78 to 743) (FIG. 4). The sequence of the mature protein (determined by computer prediction) is identical to 47.7% and 44.1% respectively with the transketolase 1 and 2 sequences of Saccharomyces cervesiae. Fewer sequence homologies were found for the other transketolases. No sequence homology was determined for the region of the transit peptide.

3. Expression analysis of the plastid transketolase

Expression analyses of some enzymes (RUBISCO, FBPase) involved in the Calvin cycle have shown that the accumulation of the corresponding transcripts is tied to green tissue and light. For monitoring the tissue-specific expression of the transketolase in tobacco plants, total RNA was isolated from sink leaves, source leaves, flower buds, stalks (internodes, nodes and cortex), roots and open flowers of growing tobacco plants. After separating in agarose gels and binding the RNA to nylon membranes, the presence of transketolase-specific transcripts was detected by hybridization with the radioactive TK-23 cDNA. As shown in FIG. 5, transketolase-specific transcripts are detectable in all organs tested. This result illustrates that in contrast to other enzymes of the Calvin cycle, transketolase in addition to its function in the Calvin cycle fulfills other objects in plant metabolism.

4. Antisense inhibition of transketolase in transgenic tobacco plants

In order to produce transgenic tobacco plants having decreased transketolase activity, the cDNA clones TK-26 and TK-28 were provided in antisense direction with a promoter effecting constitutive expression and a plant termination signal. The plasmids BinAR-anti-TK-26 and BinAR-anti-TK-28 consisting of the three fragments A, the respective cDNA (see FIG. 6, TK-26 and TK-28) and C were produced by insertion of the corresponding cDNA sequences into the expression vector pBinAR (FIG. 7A).

The fragment A comprises the 35S CaMV promoter. It contains a fragment which comprises the nucleotides 6909 to 7437 of the cauliflower mosaic virus (CaMV) (Franck et al. (1980) Cell 21, 285). It was isolated as an EcoRI-KpnI fragment from the plasmid pDH51 (Pietrzak et al. (1986) Nucleic. Acid Res. 14, 5857). The TK-26 cDNA was cloned in the pBinAR vector from the pBluescript SK (FIG. 6) as an Xbal-Sall fragment and the TK-28 cDNA as a BamHI fragment in the antisense direction (FIG. 7B and C). The fragment C contains the polyadenylation signal of the gene 3 of the T-DNA of the Ti plasmid pTiACH5 (Gielen et al. (1984); EMBO J. 3, 835), nucleotides 11749-11939, which has been isolated as a Pvull-HindIII fragment from the plasmid pAGV 40 (Herrera-Estrella et al. (1983); Nature 303,209) and after addition of SphI linkers had been cloned in the PvuII cleavage site between the SpHI-HindIII cleavage site of the vector.

The plasmids obtained were transformed in tobacco using the Agrobacterium system. Transformed tobacco plants were raised on antibiotic-containing medium and the successful inhibition of transketolase was ascertained by determination of the amount of transcript by means of Northern experiments. For each transformation (TK-26 and TK-28) 100 independent transformants were investigated. FIG. 8 shows the result of a Northern experiment. In most regenerated plants no reduction in the transketolase mRNA could be detected. Some of the plants, however, showed a greater decrease in transketolase-specific transcripts (eg. anti-TK-26 No. 26; FIG. 8). The reduction of the amount of transcript led to a suppression of plant growth. Transfer of the plants into a greenhouse led to dying off of the inhibited plants.

5. Preparation of the plasmid TK23-AC-pQE-9

For establishment of a molecular test system the plant transketolase was overexpressed in microbial systems.

For expression of the transketolase in E. coli, the TK-23 sequence, which codes for the mature polypeptide, was amplified using the primers A and C (see FIG. 9) and cloned in the vector pGEM-T (recombinant DNA methods, section 8). The TK23-AC PCR amplification product was then cloned as an SaII fragment into the SaII cleavage site of the vector PQE-9 (DIAGEN GmbH, QLAGEN Inc.) (FIG. 10).

6. Preparation of the plasmid TK23-AC-pPIC-9 and TK23-BCpHIL-D2

As eukaryotic enzymes can often only be expressed inadequately in bacterial systems, two further plasmid constructions were carried out which make possible expression in Pichia pastoris (strain GS115; Invitrogen Corporation San Diego, Calif. 92121, USA).

For secretion of the transketolase protein the plasmid TK23-AC-pPIC-9 was constructed. For fusion of the transketolase protein with a yeast signal peptide a part of the TK-23 sequence which codes for the mature polypeptide was amplified using the primers A and C (see FIG. 9) and cloned in the vector pGEM-T (recombinant DNA methods, section 8). The TK23-AC PCR amplification product was then cloned as an EcoRI fragment in the EcoRI cleavage site of the vector pPIC-9 of the Pichia expression kit (Invitrogen) (FIG. 11). In order to guarantee intracellular accumulation of the transketolase enzyme the plasmid TK23-BC-pHIL-D2 was prepared. For better purification of the enzyme a 5'-PCR primer (see FIG. 9) which contains a start codon for the translation and codes for six histidine radicals was used for amplification of the transketolase. After PCR amplification of the TK-23 sequence indicated in FIG. 9, the TK-23-BC product was cloned in the vector pGEM-T. The TK23-BC PCR amplification product was then cloned as an EcoRI fragment in the EcoRI cleavage site of the vector pHIL-D2 of the Pichia expression kit (Invitrogen) (FIG. 12).

7. Expression of the plant transketolase in E. coli

For overexpression of the transketolase in E. coli 2 ml of LB medium were inoculated with XL-1 E. coli cells which contained the plasmid TK23-AC-pQU-9. The cultures were grown overnight at 28° C. in the presence of antibiotics and with shaking. The overnight cultures were then transferred to 20 ml of growth medium (LB medium completed with: 10 μg/ml of tetracycline, 200 μg/ml of ampicillin, 1 mM vitamin B₁ and 1 mM MgSO₄). Growth took place at 28° C. with shaking. After 3 hours the expression of the transketolase was induced by addition of 2 mM IPTG. The detection of the protein produced was carried out by separating the proteins in an SDS PAAG (Laemmli (1970) Nature 227, 680-685) with subsequent Coomassie staining of the proteins. The controls used were cultures which were either not induced with IPTG or cultures which contained the transketolase in the antisense orientation. The result of an induction experiment is shown in FIG. 13. A protein of the appropriate size accumulated in bacterial cultures which were induced with IPTG and contained the plasmid TK23-AC-pQE-9. The accumulation begins an hour after induction. In the controls (without IPTG or transketolase in the antisense orientation) no comparable protein is identifiable.

FIGURES

FIG. 1 Reductive pentose phosphate cycle

FIG. 2 Linkage of the pentose phosphate cycle to other metabolic pathways

FIG. 3 Nucleotide sequence of the plastid transketolase from tobacco

FIG. 4 Amino acid comparison of the plastid transketolase with transketolase 1 and 2 from yeast

FIG. 5 Detection of the transketolase mRNA in different tobacco tissues

FIG. 6 Schematic representation of the transketolase cDNA clones

FIG. 7 Schematic representation of the plasmids BinAR-TK-26-anti and BinAR-TK-28-anu

FIG. 8 Northern analysis of transgenic tobacco plants

FIG. 9 strategy and oligonucleotides for the PCR amplification of plastid transketolase

FIG. 10 Schematic representation of the plasmid TK23-AC-pQE-9

FIG. 11 Schematic representation of the plasmid TK23-AC-pPIC-9

FIG. 12 Schematic representation of the plasmid TK23-BC-pHIL-D2

FIG. 13 Overexpression of plant transketolase in E. coli

    __________________________________________________________________________     #             SEQUENCE LISTING     - (1) GENERAL INFORMATION:     -    (iii) NUMBER OF SEQUENCES: 2     - (2) INFORMATION FOR SEQ ID NO: 1:     -      (i) SEQUENCE CHARACTERISTICS:     #pairs    (A) LENGTH: 2629 base               (B) TYPE: nucleic acid               (C) STRANDEDNESS: single               (D) TOPOLOGY: linear     -     (ii) MOLECULE TYPE: cDNA     -    (iii) HYPOTHETICAL: No     -     (vi) ORIGINAL SOURCE:               (A) ORGANISM: Nicotiana     -     (ix) FEATURE:               (A) NAME/KEY: CDA               (B) LOCATION: 60..2289     #SEQ ID NO: 1:) SEQUENCE DESCRIPTION:     - CTCCTCTTCA CTCTCTTTTC TCTTTGAGAC AAAACATCAA ACACCTTACT GG - #TAAAGCC     59     - ATG GCG TCT TCT TCT TCT CTC ACT CTC TCT CA - #A GCT ATC CTC TCT CGT     107     Met Ala Ser Ser Ser Ser Leu Thr Leu Ser Gl - #n Ala Ile Leu Ser Arg     #                 15     - TCT GTC CCT CGC CAT GGC TCT GCC TCT TCT TC - #T CAA CTT TCC CCT TCT     155     Ser Val Pro Arg His Gly Ser Ala Ser Ser Se - #r Gln Leu Ser Pro Ser     #             30     - TCT CTC ACT TTT TCC GGC CTT AAA TCC AAT CC - #C AAT ATC ACC ACC TCC     203     Ser Leu Thr Phe Ser Gly Leu Lys Ser Asn Pr - #o Asn Ile Thr Thr Ser     #         45     - CGC CGC CGT ACT CCT TCC TCC GCC GCC GCC GC - #C GCC GTC GTA AGG TCA     251     Arg Arg Arg Thr Pro Ser Ser Ala Ala Ala Al - #a Ala Val Val Arg Ser     #     60     - CCG GCG ATT CGT GCC TCA GCT GCA ACC GAA AC - #C ATA GAG AAA ACT GAG     299     Pro Ala Ile Arg Ala Ser Ala Ala Thr Glu Th - #r Ile Glu Lys Thr Glu     # 80     - ACT GCG CTT GTT GAC AAA TCT GTA AAC ACG AT - #T CGA TTT TTG GCT ATT     347     Thr Ala Leu Val Asp Lys Ser Val Asn Thr Il - #e Arg Phe Leu Ala Ile     #                 95     - GAT GCT GTT GAA AGG CAA ATT CGG GTC ACC CG - #G TTT GCC ATG GGT TGT     395     Asp Ala Val Glu Arg Gln Ile Arg Val Thr Ar - #g Phe Ala Met Gly Cys     #           110     - GCT CCG ATG GGT CAT ATA TTG TAC GAT GAG GT - #T ATG AGG TAT AAC CCG     443     Ala Pro Met Gly His Ile Leu Tyr Asp Glu Va - #l Met Arg Tyr Asn Pro     #       125     - AAA AAC CCG TAT TGG TTT AAT CGG GAT CGG TT - #T GTT CTA TCA GCT GGA     491     Lys Asn Pro Tyr Trp Phe Asn Arg Asp Arg Ph - #e Val Leu Ser Ala Gly     #   140     - CAT GGT TGT ATG CTT CAG TAT GCT TTG CTT CA - #T CTA GCT GGC TAT GAT     539     His Gly Cys Met Leu Gln Tyr Ala Leu Leu Hi - #s Leu Ala Gly Tyr Asp     145                 1 - #50                 1 - #55                 1 -     #60     - GCT GTC AGG GAA GAG GAC TTG AAG AGC TTC CG - #T CAG TGG GGA ACC AAA     587     Ala Val Arg Glu Glu Asp Leu Lys Ser Phe Ar - #g Gln Trp Gly Thr Lys     #               175     - ACC CCT GGA CAC CCT GAA AAC TTT GAG ACA CC - #T GGT GTT GAA GTC ACC     635     Thr Pro Gly His Pro Glu Asn Phe Glu Thr Pr - #o Gly Val Glu Val Thr     #           190     - ACC GGG CCT CTG GGA CAA GGT ATT GCC AAC GC - #C GTT GGC TTG GCT CTT     683     Thr Gly Pro Leu Gly Gln Gly Ile Ala Asn Al - #a Val Gly Leu Ala Leu     #       205     - GTG GAG AAA CAC TTG GCT GCT CGT TTC AAT AA - #G CCT GAC GCT GAG ATT     731     Val Glu Lys His Leu Ala Ala Arg Phe Asn Ly - #s Pro Asp Ala Glu Ile     #   220     - GTA GAC CAC TAC ACA TAT GTT ATT CTC GGT GA - #T GGT TGC CAG ATG GAG     779     Val Asp His Tyr Thr Tyr Val Ile Leu Gly As - #p Gly Cys Gln Met Glu     225                 2 - #30                 2 - #35                 2 -     #40     - GGT ATT TCA CAA GAA GCT TGT TCC CTT GCT GG - #A CAC TGG GGA CTT GGA     827     Gly Ile Ser Gln Glu Ala Cys Ser Leu Ala Gl - #y His Trp Gly Leu Gly     #               255     - AAG CTG ATT GCT TTC TAT GAT GAC AAC CAC AT - #C TCA ATT GAT GGT GAC     875     Lys Leu Ile Ala Phe Tyr Asp Asp Asn His Il - #e Ser Ile Asp Gly Asp     #           270     - ACA GAA ATC GCT TTC ACT GAG GAT GTT GGT GC - #C CGT TTT GAG GCT CTT     923     Thr Glu Ile Ala Phe Thr Glu Asp Val Gly Al - #a Arg Phe Glu Ala Leu     #       285     - GGG TGG CAC GTA ATC TGG GTG AAG AAC GGT AA - #C ACT GGT TAT GAT GAG     971     Gly Trp His Val Ile Trp Val Lys Asn Gly As - #n Thr Gly Tyr Asp Glu     #   300     - ATT CGT GCT GCT ATT AAG GAA GCA AAA ACT GT - #C ACA GAC AAA CCC ACT     1019     Ile Arg Ala Ala Ile Lys Glu Ala Lys Thr Va - #l Thr Asp Lys Pro Thr     305                 3 - #10                 3 - #15                 3 -     #20     - ATG ATC AAG GTG ACT ACA ACC ATT GGT TTT GG - #C TCG CCC AAC AAG GCA     1067     Met Ile Lys Val Thr Thr Thr Ile Gly Phe Gl - #y Ser Pro Asn Lys Ala     #               335     - AAC AGT TAC AGT GTA CAT GGA AGT GCA CTT GG - #A GCT AAG GAA GTA GAG     1115     Asn Ser Tyr Ser Val His Gly Ser Ala Leu Gl - #y Ala Lys Glu Val Glu     #           350     - GCC ACC AGG AGT AAC TTG GGA TGG CCT TAT GA - #G CCT TTC CAT GTG CCT     1163     Ala Thr Arg Ser Asn Leu Gly Trp Pro Tyr Gl - #u Pro Phe His Val Pro     #       365     - GAA GAT GTC AAG AGC CAT TGG AGT CGT CAT GT - #T CCC GAG GGT GCT GCT     1211     Glu Asp Val Lys Ser His Trp Ser Arg His Va - #l Pro Glu Gly Ala Ala     #   380     - CTT GAA GCT GGG TGG AAT ACC AAG TTT GCT GA - #A TAT GAG AAG AAG TAC     1259     Leu Glu Ala Gly Trp Asn Thr Lys Phe Ala Gl - #u Tyr Glu Lys Lys Tyr     385                 3 - #90                 3 - #95                 4 -     #00     - CCA GAG GAA GCT GCA GAA CTC AAA TCC ATT AC - #T ACT GGT GAA CTA CCT     1307     Pro Glu Glu Ala Ala Glu Leu Lys Ser Ile Th - #r Thr Gly Glu Leu Pro     #               415     - GCT GGC TGG GAG AAA GCT CTT CCT ACC TAC AC - #A CCT GAA AGT CCA GCG     1355     Ala Gly Trp Glu Lys Ala Leu Pro Thr Tyr Th - #r Pro Glu Ser Pro Ala     #           430     - GAT GCC ACC AGA AAC CTG TCC CAA CAA AAC CT - #G AAT GCT CTT GCC AAG     1403     Asp Ala Thr Arg Asn Leu Ser Gln Gln Asn Le - #u Asn Ala Leu Ala Lys     #       445     - GTT CTT CCT GGT TTC CTT GGT GGT AGT GCT GA - #T CTT GCC TCA TCA AAC     1451     Val Leu Pro Gly Phe Leu Gly Gly Ser Ala As - #p Leu Ala Ser Ser Asn     #   460     - ATG ACC CTC ATG AAA ATG TTT GGT GAC TTC CA - #A AAG AAC ACC CCA GAG     1499     Met Thr Leu Met Lys Met Phe Gly Asp Phe Gl - #n Lys Asn Thr Pro Glu     465                 4 - #70                 4 - #75                 4 -     #80     - GAG CGT AAT CTA AGG TTT GGT GTT CGT GAA CA - #T GGT ATG GGA GCC ATA     1547     Glu Arg Asn Leu Arg Phe Gly Val Arg Glu Hi - #s Gly Met Gly Ala Ile     #               495     - TGT AAT GGT AAT GCT CTA CAC AGC CCT GGC TT - #G ATT CCC TAC TGT GCT     1595     Cys Asn Gly Asn Ala Leu His Ser Pro Gly Le - #u Ile Pro Tyr Cys Ala     #           510     - ACT TTC TTT GTG TTC ACC GAC TAC ATG AGA GG - #A GCT ATG AGA ATT TCA     1643     Thr Phe Phe Val Phe Thr Asp Tyr Met Arg Gl - #y Ala Met Arg Ile Ser     #       525     - GCC TTG TCT GAG GCT GGA GTT ATT TAT GTT AT - #G ACC CAC GAT TCA ATT     1691     Ala Leu Ser Glu Ala Gly Val Ile Tyr Val Me - #t Thr His Asp Ser Ile     #   540     - GGT CTA GGA GAA GAT GGG CCT ACC CAT CAA CC - #C ATT GAG CAC TTG CCA     1739     Gly Leu Gly Glu Asp Gly Pro Thr His Gln Pr - #o Ile Glu His Leu Pro     545                 5 - #50                 5 - #55                 5 -     #60     - AGT TTC CGT GCA ATG CCC AAC ATT CTG ATG TT - #C CGT CCA GCA GAT GGC     1787     Ser Phe Arg Ala Met Pro Asn Ile Leu Met Ph - #e Arg Pro Ala Asp Gly     #               575     - AAG GAG ACA GCG GGA GCT TAC AAG GTG GCT GT - #C CTC AAG AGG AAG ACA     1835     Lys Glu Thr Ala Gly Ala Tyr Lys Val Ala Va - #l Leu Lys Arg Lys Thr     #           590     - CCA TCA ATC CTT GCC CTC TCT CGG CAA AAG TT - #G CCA CAA CTT GCT GGA     1883     Pro Ser Ile Leu Ala Leu Ser Arg Gln Lys Le - #u Pro Gln Leu Ala Gly     #       605     - AGT TCT ATT GAA GGA GCA GCA AAG CGT GGC TA - #C ATT TTA TCA GAC AAT     1931     Ser Ser Ile Glu Gly Ala Ala Lys Arg Gly Ty - #r Ile Leu Ser Asp Asn     #   620     - TCT TCT GGC AAC AAA CCT GAT GTC ATT TTG AT - #T GGT ACT GGC TCA GAG     1979     Ser Ser Gly Asn Lys Pro Asp Val Ile Leu Il - #e Gly Thr Gly Ser Glu     625                 6 - #30                 6 - #35                 6 -     #40     - TTA GAA ATT GCT GTC AAG GCT GCT GAT GAA CT - #C AGG AAA GAA GGA AAA     2027     Leu Glu Ile Ala Val Lys Ala Ala Asp Glu Le - #u Arg Lys Glu Gly Lys     #               655     - GCA GTG AGA GTT GTT TCC TTT GTT TGT TGG GA - #G CTT TTT GAA GAA CAA     2075     Ala Val Arg Val Val Ser Phe Val Cys Trp Gl - #u Leu Phe Glu Glu Gln     #           670     - TCA GCC GAC TAC AAG GAA AGT GTC CTT CCA TC - #A TCT GTT ACA GCT AGA     2123     Ser Ala Asp Tyr Lys Glu Ser Val Leu Pro Se - #r Ser Val Thr Ala Arg     #       685     - GTT AGC ATT GAG GCC GGA TCC ACA TTT GGG TG - #G GAG AAA TAT GTC GGA     2171     Val Ser Ile Glu Ala Gly Ser Thr Phe Gly Tr - #p Glu Lys Tyr Val Gly     #   700     - TCA AAG GGG AAG GCC ATC GGA ATT GAC AGA TG - #G GGT GCC AGT GCC CCT     2219     Ser Lys Gly Lys Ala Ile Gly Ile Asp Arg Tr - #p Gly Ala Ser Ala Pro     705                 7 - #10                 7 - #15                 7 -     #20     - GCT GGA AAA ATA TAC AAG GAG TAC GGA ATT AC - #A GCA GAG GCT GTT GTA     2267     Ala Gly Lys Ile Tyr Lys Glu Tyr Gly Ile Th - #r Ala Glu Ala Val Val     #               735     - GCT GCA GCT AAA CAA GTT TCT T AGGCTTTATT ACTT - #ACCCTT GGTTGCTGGT     2319     Ala Ala Ala Lys Gln Val Ser                 740     - GTCTACCAAA TTTGTTTTCA TTTTGAAACT GAGGTTGGAG ATAACGGTGG AA - #ACCAATAC     2379     - CAAACGGACT CGGCAGTTCA CTGTTGCCTG GTATTTTCAA TAAAAACTAT TT - #CTTCATCT     2439     - GTCCTTTGTT TTCTTCAGTT TTAGTAGCGG AGCGGCCAAA ATGAATCCAA GA - #TGAGGATA     2499     - GAAATAGGAT TATGGATGCT CCTGACCATG TACACTTAAA ACATATCTGT GA - #GTTTTGTA     2559     - ATTTTATTTG GTCGAGTGAT ACCAAGATCT CATTTTCAAT TGGAAAAAAA AA - #AAAAAAAA     2619     #    2629     - (2) INFORMATION FOR SEQ ID NO: 2:     -      (i) SEQUENCE CHARACTERISTICS:     #acids    (A) LENGTH: 743 amino               (B) TYPE: amino acid               (D) TOPOLOGY: linear     -     (ii) MOLECULE TYPE: protein     #SEQ ID NO: 2:) SEQUENCE DESCRIPTION:     - Met Ala Ser Ser Ser Ser Leu Thr Leu Ser Gl - #n Ala Ile Leu Ser Arg     #                 15     - Ser Val Pro Arg His Gly Ser Ala Ser Ser Se - #r Gln Leu Ser Pro Ser     #             30     - Ser Leu Thr Phe Ser Gly Leu Lys Ser Asn Pr - #o Asn Ile Thr Thr Ser     #         45     - Arg Arg Arg Thr Pro Ser Ser Ala Ala Ala Al - #a Ala Val Val Arg Ser     #     60     - Pro Ala Ile Arg Ala Ser Ala Ala Thr Glu Th - #r Ile Glu Lys Thr Glu     # 80     - Thr Ala Leu Val Asp Lys Ser Val Asn Thr Il - #e Arg Phe Leu Ala Ile     #                 95     - Asp Ala Val Glu Arg Gln Ile Arg Val Thr Ar - #g Phe Ala Met Gly Cys     #           110     - Ala Pro Met Gly His Ile Leu Tyr Asp Glu Va - #l Met Arg Tyr Asn Pro     #       125     - Lys Asn Pro Tyr Trp Phe Asn Arg Asp Arg Ph - #e Val Leu Ser Ala Gly     #   140     - His Gly Cys Met Leu Gln Tyr Ala Leu Leu Hi - #s Leu Ala Gly Tyr Asp     145                 1 - #50                 1 - #55                 1 -     #60     - Ala Val Arg Glu Glu Asp Leu Lys Ser Phe Ar - #g Gln Trp Gly Thr Lys     #               175     - Thr Pro Gly His Pro Glu Asn Phe Glu Thr Pr - #o Gly Val Glu Val Thr     #           190     - Thr Gly Pro Leu Gly Gln Gly Ile Ala Asn Al - #a Val Gly Leu Ala Leu     #       205     - Val Glu Lys His Leu Ala Ala Arg Phe Asn Ly - #s Pro Asp Ala Glu Ile     #   220     - Val Asp His Tyr Thr Tyr Val Ile Leu Gly As - #p Gly Cys Gln Met Glu     225                 2 - #30                 2 - #35                 2 -     #40     - Gly Ile Ser Gln Glu Ala Cys Ser Leu Ala Gl - #y His Trp Gly Leu Gly     #               255     - Lys Leu Ile Ala Phe Tyr Asp Asp Asn His Il - #e Ser Ile Asp Gly Asp     #           270     - Thr Glu Ile Ala Phe Thr Glu Asp Val Gly Al - #a Arg Phe Glu Ala Leu     #       285     - Gly Trp His Val Ile Trp Val Lys Asn Gly As - #n Thr Gly Tyr Asp Glu     #   300     - Ile Arg Ala Ala Ile Lys Glu Ala Lys Thr Va - #l Thr Asp Lys Pro Thr     305                 3 - #10                 3 - #15                 3 -     #20     - Met Ile Lys Val Thr Thr Thr Ile Gly Phe Gl - #y Ser Pro Asn Lys Ala     #               335     - Asn Ser Tyr Ser Val His Gly Ser Ala Leu Gl - #y Ala Lys Glu Val Glu     #           350     - Ala Thr Arg Ser Asn Leu Gly Trp Pro Tyr Gl - #u Pro Phe His Val Pro     #       365     - Glu Asp Val Lys Ser His Trp Ser Arg His Va - #l Pro Glu Gly Ala Ala     #   380     - Leu Glu Ala Gly Trp Asn Thr Lys Phe Ala Gl - #u Tyr Glu Lys Lys Tyr     385                 3 - #90                 3 - #95                 4 -     #00     - Pro Glu Glu Ala Ala Glu Leu Lys Ser Ile Th - #r Thr Gly Glu Leu Pro     #               415     - Ala Gly Trp Glu Lys Ala Leu Pro Thr Tyr Th - #r Pro Glu Ser Pro Ala     #           430     - Asp Ala Thr Arg Asn Leu Ser Gln Gln Asn Le - #u Asn Ala Leu Ala Lys     #       445     - Val Leu Pro Gly Phe Leu Gly Gly Ser Ala As - #p Leu Ala Ser Ser Asn     #   460     - Met Thr Leu Met Lys Met Phe Gly Asp Phe Gl - #n Lys Asn Thr Pro Glu     465                 4 - #70                 4 - #75                 4 -     #80     - Glu Arg Asn Leu Arg Phe Gly Val Arg Glu Hi - #s Gly Met Gly Ala Ile     #               495     - Cys Asn Gly Asn Ala Leu His Ser Pro Gly Le - #u Ile Pro Tyr Cys Ala     #           510     - Thr Phe Phe Val Phe Thr Asp Tyr Met Arg Gl - #y Ala Met Arg Ile Ser     #       525     - Ala Leu Ser Glu Ala Gly Val Ile Tyr Val Me - #t Thr His Asp Ser Ile     #   540     - Gly Leu Gly Glu Asp Gly Pro Thr His Gln Pr - #o Ile Glu His Leu Pro     545                 5 - #50                 5 - #55                 5 -     #60     - Ser Phe Arg Ala Met Pro Asn Ile Leu Met Ph - #e Arg Pro Ala Asp Gly     #               575     - Lys Glu Thr Ala Gly Ala Tyr Lys Val Ala Va - #l Leu Lys Arg Lys Thr     #           590     - Pro Ser Ile Leu Ala Leu Ser Arg Gln Lys Le - #u Pro Gln Leu Ala Gly     #       605     - Ser Ser Ile Glu Gly Ala Ala Lys Arg Gly Ty - #r Ile Leu Ser Asp Asn     #   620     - Ser Ser Gly Asn Lys Pro Asp Val Ile Leu Il - #e Gly Thr Gly Ser Glu     625                 6 - #30                 6 - #35                 6 -     #40     - Leu Glu Ile Ala Val Lys Ala Ala Asp Glu Le - #u Arg Lys Glu Gly Lys     #               655     - Ala Val Arg Val Val Ser Phe Val Cys Trp Gl - #u Leu Phe Glu Glu Gln     #           670     - Ser Ala Asp Tyr Lys Glu Ser Val Leu Pro Se - #r Ser Val Thr Ala Arg     #       685     - Val Ser Ile Glu Ala Gly Ser Thr Phe Gly Tr - #p Glu Lys Tyr Val Gly     #   700     - Ser Lys Gly Lys Ala Ile Gly Ile Asp Arg Tr - #p Gly Ala Ser Ala Pro     705                 7 - #10                 7 - #15                 7 -     #20     - Ala Gly Lys Ile Tyr Lys Glu Tyr Gly Ile Th - #r Ala Glu Ala Val Val     #               735     - Ala Ala Ala Lys Gln Val Ser                 740     __________________________________________________________________________ 

We claim:
 1. An isolated plastidic transketolase, comprising a fragment of at least 100 amino acids from SEQ ID NO 2, wherein said plastidic transketolase is free of cytoplasmic transketolase.
 2. The plastidic transketolase of claim 1, comprising a fragment of SEQ ID NO:2 from amino acid 78 to amino acid
 743. 3. The plastidic transketolase of claim 2, comprising an amino acid sequence shown in SEQ ID NO
 2. 4. A method of identifying a transketolase inhibitor, which method comprises incubating a potential transketolase inhibitor with the protein defined in claim 1, and then determining the level of transketolase activity of the protein, said transketolase inhibitor being identified by a decrease in transketolase activity of the protein. 